1. Field of the Invention
Agricultural byproducts such as sugar beet pulp, citrus fruit pulp, seed hulls, and cereal bran exiting as waste streams from commercial crop processing plants have little inherent value and have traditionally constituted a disposal problem. These materials represent an abundant, inexpensive, and readily available source of renewable lignocellulosic biomass. However, their utilization as a carbohydrate source for glucose and ethanol production, and as a metabolic energy source in ruminant feeds, has been severely hampered by the low efficiency with which organisms and enzymes are able to convert the polysaccharide portion of the residue into monomeric sugars. The low conversion efficiency for lignocellulosic materials is the result of two principal factors: (1) unavailability of the cellulose and hemicellulose resulting from the close physical and chemical association between lignin and these polysaccharides in the plant cell wall, and (2) the degree of crystallinity within the cellulose polymer itself.
Lignin is thought to prevent the degradation of cellulose mainly by acting as a physical barrier between the cellulolytic enzyme and its substrate. Consequently, the rate and extent of enzymatic cellulose degradation in lignocellulosic materials is inversely related to the lignin content, with maximum degradation occurring only after 50% or more of the lignin has been removed. Even when lignin levels are low, however, the hydrolysis of cellulose can be limited by the physical properties of the polysaccharide itself. Amorphous regions of cellulose are hydrolyzed at much higher rates than are microcrystalline regions, for example.
This invention relates to a pretreatment process for rendering the polysaccharide components of nonwoody lignocellulosic residues available for use in biological systems as sources of carbohydrate.
2. Description of the Prior Art
Numerous pretreatments have been developed in an effort to increase the efficiency of enzymatic saccharification. These processes utilize physical, chemical, and/or biological methods to remove lignin and decrease cellulose crystallinity. Although most of these pretreatments do result in increased cellulose hydrolysis, the yields of glucose obtained are usually still well below theoretical levels. Moreover, processes such as autohydrolysis, alkaline cooking, and steam explosion require substantial energy input in the form of heat and tend to generate toxic side products. A few pretreatments have been developed that allow essentially quantitative conversion of cellulose into glucose, but these processes involve the use of expensive, highly toxic reagents such as cadoxen, ethylenediamine, or peracetic acid. Toxic constituents in the digest, of course, interfere with subsequent biological saccharification and fermentation steps, and also prohibit use of the digest as an animal feed. Other drawbacks typical of conventional pretreatments include loss of the hemicellulose with the solubilized fraction and also reversion of the cellulose crystallinity upon drying.
In nature, lignin is degraded by various organisms, primarily to increase the amount of cellulose available for enzymatic digestion. Although the mechanism of natural lignin degradation is largely unknown, it is thought that oxidants such as H.sub.2 O.sub.2 may play an important role [Tien et al., Science 221: 661-662 Aug. 12, 1983)]. Hydrogen peroxide is known to react with lignin under certain conditions and has been widely used for many years to bleach high-lignin wood pulps. More recently, Lachenal et al. [Tappi 63(4): 119-122 (April 1980)] have found that at 80.degree.-120.degree. C. under alkaline conditions, H.sub.2 O.sub.2 will delignify kraft pulps with partial degradation of the cellulose. However, failure to preserve the hemicellulose fraction and the requisite for high energy and reagent inputs diminish the appeal of applying the Lachenal et al. process to agricultural residues. Takagi (U.S. Pat. No. 4,314,854) observed that when cellulosic materials were treated with a combination of H.sub.2 O.sub.2 and manganese salts, cellulosic materials were somewhat more susceptible to hydrolysis by cellulolytic enzymes but with no apparent advantage derived from the hemicellulose.
Dreyfus (U.S. Pat. No. 2,487,114) obtains a product high in alpha-cellulose content and low in hemicellulose by treating straw with hydrogen peroxide in highly alkaline aqueous solution. In the parent application referred to above, and incorporated herein by reference, a process is disclosed for efficiently converting leaves and stalks of nonwoody plants to useful carbohydrate sources for ruminants and microbes by treatment with alkaline peroxide. The treatment is under conditions of controlled pH which are designed to conserve the hemicellulose in the recovered carbohydrate fraction. Illustrative substrates useful in the process include agricultural residues and noncultivated grasses such as wheat straw, corn stalks, corn cobs, and prairie grass.